Multi-stage free-piston compressor



MULTI-STAGE FREE-PISTON COMPRESSOR Filed Jan. 11 1963 2 Sheets-Sheet 1 Fig. 1

L-1- N l l 1965 H. JANICKE 3,200,249

MULTI-STAGE FREE-PI 8 TON COMPRE S S OR Filed Jan. 11 1963 2 Sheets-Sheet 2 Fig. 4

United States Patent 0 4 Claims. C1. zenss The invention is concerned with improvements relating to multi-stage free-piston compressors. Such compressors may be used for example for conveying natural gas over long distances, the gas also serving as the fuel for driving the compressor.

The invention is a development of the multi stage freepiston compressors described in my patent application Serial No. 37,745 filed June 21, 1960, now Patent No. 3,125,282. In such compressors, the gas to be compressed is supplied at a pressure which may vary, the pressure being above atmospheric pressure, and in order to govern engine compression i.e. compression of the gaseous fuel for driving the compressor, the rear side of the compressor piston is loaded with a prevailing operating pressure of the compressor such as the admission pressure of the gas to the compressor or the delivery pressure from the compressor.

I It has been shown that loading the rear side of a first stage compressor piston of a multistage compressor with the admission pressure does not give the desired effect. This is due to the fact that with the higher overall pressure ratio of multi-stage machines, the diameter of the first stage becomes smaller so that there is no longer any possibility of compensating for the increase in the work done on. the compressor pistons of the remaining stages during the inward stroke by loading the rear side of the first stage compressor piston with the admission pressure.

With, for example, a two-stage free-piston compressor there is the obvious possibility of arranging the first and second compressor stages so that they compress during the outward stroke, and loading the rear side of the first stage compressor piston with the intermediate pressure, that is, with the delivery pressure of the first compressor stage. This cannot however produce the desired result since with multi-stage compressors the intermediate pressure varies proportionately much less when the admission pressure to the first stage changes, so that with rising admission pressure to the first stage there results an inadmissably high rise of the work available for compressing the fuel for driving the compressor.

Another apparent solution would be to have the first compressor stage compressing during the inward stroke and the second compressor stage compressing during the outward stroke. Here, the work done on the second stage during the inward stroke must be and remain, larger than the work done by the first stage during the inward stroke by the amount of work required for compressing the fuel for driving the compressor, plus any remaining work such as that expended in overcoming friction. A more detailed examination shows that the work done by the first stage during the inward stroke rises more strongly with rising admission pressure than the work done on the second stage during the inward stroke, so that the work available during the inward stroke for compressing the engine fuel becomes too small. The reasons are as follows. In order to obtain a sulficient amount of available work during the inward stroke for compressing the engine fuel, the piston areas of the two stages must not be in inverse proportion to the pressures, as is usually the case, but the second stage must be given a disproportion ately larger piston area. This results in a comparatively larger dead space, and hence a lower volumetric efficiency. Assuming that for the different pressures with which the gas may be supplied to the compressor, the latter is to be operated with the same amount of fuel, corresponding to full load, there results necessarily a smaller gas admission volume, that is to say, the stroke becomes smaller when the supply pressure rises. The admission volume of the second stage, compressing outwardly, and having a comparatively larger dead space, would be, with the same intermediate pressure, smaller than the delivery volume of the first stage, compressing inwardly the dead space of which remains substantially constant. The increase in the intermediate pressure, producing the equalization, exceeds here the normal value, resulting in an excessive reduction of the total work available for compressing engine fuel during the inward stroke.

The invention has therefore the object of keeping the work available for compressing engine fuel Within permissible limits when the admission pressure varies.

From another point of view, the object of the invention might be regarded as providing a multi-stage freepiston compressor with which the effect of variations in the admission pressure of gas to the compressor on the work done in compressing gaseous fuel for driving the engine is reduced.

According to this invention, this object is realized in that the first stage of the compressor is double-acting and the remaining stages single-acting, and that in the first stage the efforts are so distributed by dim nsioning the effective piston areas that the intermediate pressure, dependent on the distribution, results with changes in the admission pressure so that the difference between the work done, during the inward stroke, on the stages which compress during the outward stroke, and the work done, during the inward stroke by the part of the first stage which compresses during the inward stroke, leaves a resultant amount of work available which is for cornpressing engine fuel during the inward stroke.

The invention enables the work available for compressing engine fuel to be kept within permissible limits where changes in the delivery pressure occur, in addition to changes in the admission pressure.

Further advantages and features of the invention will become apparent from the following description of an embodiment with reference to the accompanying drawings. In the drawings:

FIGURE 1 is the right hand half of a diagrammatic cross-sectional view of a two-stage free-piston compressor which is substantially symmetrical on either side of its central transverse axis 5, and

FIGURES 2 to 4 are diagrams relating pressure (ordinate) and piston stroke (abscissa) for a two-stage compressor according to the invention, with different admis sion pressures and constant delivery pressure.

in FIG. 1, part of the left half of engine cylinder 1 is indicated in order to clarify the operation of the compressor. In the engine cylinder, two engine pistons 3a and 3b are axially slidable. The right piston 3a controls scavenging slots 2a, whilst the left piston 31) controls exhaust ports 21) located at a smaller distance from the line S than the scavenging ports 25!. Through the scav enging ports 2a, scavenging air may be blown into the cylinder 1 by a scavenging blower 2t), when the piston 3a opens the scavenging ports 2a. The blower 20 may be driven by a driving motor 21, e.g., a turbine operated by exhaust gases from the engine cylinder. The supply, preferably in gaseous form, of fuel to the cylinder ll may be effected by a metering device 22, shown diagrammatically in the drawing, and having preferably the design disclosed in Patent No. 3,043,282, filed July 28, 1960. In a preferred application, the compressor is used for conveying combustible gases, e.g., natural gases over long distances and FIG. 1 shows a conduit 23 for feeding gas which has been tapped from a delivery conduit (not shown) to the metering device 22.

The piston 3a forms together with a first stage piston and a second stage piston 6, a right hand piston assembly. The piston 3a is connected by means of a piston rod 4 to the piston 5 which moves in a cylinder 24. The piston rod 4 runs in a seal 25 which seals one side of the cylinder 24. The piston 5 is connected to the piston 6 which is of smaller diameter than the piston 5, and which moves in a cylinder 26 of the second stage. As may be seen, the piston 6 is the piston of a single-acting pumptype second compressor stage, and compresses gas in cylinder chamber 7 during outward movement of the piston 3a, i.e., away from the axis S.

The piston 5 is the piston of a double-acting pump-type first compressor stage, and compresses gas in cylinder chamber 8 during inward movement of the piston 3a, and gas in cylinder chamber 9 during the outward stroke of the piston 3a.

The cylinder 24 has inlets with admission valves 10a and 1012 leading to the cylinder chambers 8 and 9 respectively. In addition, there are provided outlets from the chambers 8 and 9, the outlets being fitted with pressure valves 11a and 11b respectively. The inlets equipped with the inlet valves 10a and 10b are supplied through a conduit 14 with the gas to be compressed, at a supply pressure which may vary. Gas compressed in the chambers 8 and 9 passes the valves 11a, 11b, and flows along conduits into an intermediate cooler 15. From this cooler the gas passes an inlet valve 12 to the cylinder chamber 7, is there further compressed by the piston 6, and is delivered through pressure valve 13 at the delivery pressure from the compressor. Practical dimensions for the piston assembly are as follows:

Diam., mm. Piston 3a 370 Piston rod 4 120 Piston 5 270 Piston6 165 From these data, the effective piston areas may be calculated as follows: the left side of piston 5 facing cylinder chamber 8: 459.46 cm. the right side of piston 5 facing the chamber 9: 358.74 cm. the piston area of piston 6: 213.82 cm.

The diagrams of FIGS. 2 to 4 show qualitatively and quantitatively the conditions obtaining with a two-stage compressor according to FIG. 1, wherein the admission pressure to the double-acting first compressor stage, represented by the lines 1-7 and 2-3 differs in all three examples, whilst the delivery pressure, represented by the line 212, is constant in all the examples. The abscissa shows the piston stroke, the arrow H indicating an outward piston stroke and the arrow H an inward piston stroke.

The diagrams indicate changes in piston stroke, intermediate pressure, admission volume and changes in the work in the individual compressor stages. The small deviations of the intermediate pressure caused by the finite size of the intermediate cooler 15 are negligibly small and are not shown in the diagrams. Assuming that a constant amount of fuel is fed to the engine cylinder, the piston stroke adjusts itself automatically to the rele vant value shown in the diagrams.

In the diagrams, the line 1-2 indicates the stroke; the lines 78-91)7 show the diagram for chamber 9 of the first compressor stage; the lines 51112-135 show the diagram for the chamber 7 of the second compressor stage; and the lines 345-6-3 show the diagram for chamber 8 of the first compressor stage. The diagrams correspond to actually obtainable conditions, because the work done in compressing the fuel for driving the compressor could be kept within permissible limits.

With the selected example, the admission pressure amounted in one case (FIG. 2) to 9 kg./cm. resulting in a piston stroke of 450 mm.; in a second case (FIG. 3), it was 15 kg./cm. and the resulting piston stroke was 400 mm.; and in a third case (FIG. 4) it was 25 kg./cm. resulting in a piston stroke of 365 mm. The delivery pressure, given by the line 2-12 was in all three cases 67 kg./cm.

The work available for compressing the fuel for driving the compressor can be calculated from the result of adding the area of 1-511-12-2-1 to the area of 1-7- 8-9-2-1 and subtracting the area of 12-3-4-5-1.

I claim:

1. A two-stage free-piston compressor for compressing gas supplied thereto under an admission pressure which may vary, including:

a reciprocable free-piston assembly comprised of:

an engine cylinder;

left and right-hand engine pistons slidable within said engine cylinder;

means for supplying gaseous fuel for driving the compressor to said engine cylinder;

each of said engine pistons being coupled to:

a first compressor stage comprising a first doublefaced piston for compressing gas on both sides thereof;

and a second compressor stage comprising a second piston;

means for feeding said gas being supplied thereto under pressure which may vary to said first compressor stage;

means for coupling the output of both sides of said first compressor stage to said second compressor stage;

the piston area of both faces of said first piston each being greater than the piston area of said second piston causing the gas compressed therein to exert a resultant force on said piston areas for effecting compression of said gaseous fuel in the engine cylinder, and simultaneously for effecting the induction stroke of said second compressor stage.

2. A multi-stage free-piston compressor for compressing gas supplied thereto at an admission pressure which may vary, said compressor comprising:

a reciprocable free-piston assembly, said assembly comprising first and second engine pistons and an engine cylinder in which said pistons are slidable, and means for supplying gaseous fuel for driving the compressor to said engine cylinder;

each of said engine pistons being coupled to a plurality of compressor stages;

one of said compressor stages being a double-acting pump-type compressor stage having a first doublefaced piston for compressing gas on both sides thereof;

at least one of said remaining stages being a singleacting pump-type compressor stage having a second piston;

the output of both sides of said double-acting stage being coupled to said single-acting compressor stage;

the piston area of said double-acting stage being substantially greater than the piston area of said singleacting compressor stage causing the gas being fed thereto to exert a resultant force on said pistons to effect compression of said gaseous fuel in the engine cylinder, and simultaneously to effect the induction stroke of the single-acting compressor stage.

3. A multi-stage free-piston compressor for compressing gas supplied thereto at an admission pressure which may vary, wherein said compressor comprises:

a central engine cylinder;

means for supplying said engine cylinder with gaseous fuel for driving the compressor;

two synchronously operable free-piston assemblies extending outwardly from opposite ends of said engine cylinder, each piston assembly including:

5 an engine piston slidable in said engine cylinder, a first-stage double-faced piston, and a second stage piston;

a first double-acting pump-type compressor stage, said first-stage piston being employed in said first compressor stage for compressing gas on both sides thereof; I

a second single-acting pump-type compressor stage, said second-stage piston being employed in said second compressor stage;

means for feeding the gas supplied thereto at an admission pressure which may vary into said first compressor stage;

means coupling the output of both sides of said first compressor stage to said second compressor stage;

the piston area of said first compressor stage being substantially greater than the piston area of said second compressor stage causing the gas compressed therein to exert a resultant force on each piston assembly for efiecting compression of said gaseous fuel between the engine pistons, and simultaneously for eifecting the induction stroke of each piston assembly for said second compressor stage.

4. A compressor of the type described in claim 3, in Which each of said piston assemblies is comprised of a piston rod coupling the engine piston to the first-stage double-faced piston; said second stage piston being a cylindrical member having a first end thereof being connected to said first-stage piston; the diameter of said cylindrical member being less than the diameter of said first-stage piston and being greater than the diameter of said piston rod; the piston areas of said double-faced first-stage piston being unequal and being greater than the piston area of said second-stage piston.

References Cited by the Examiner 2,667,300 1/54 Huber 230-56 FOREIGN PATENTS 1,139,065 6/57 France.

LAURENCE V. EFNER, Primary Examiner.

ROBERT M. WALKER, Examiner. 

1. A TWO-STAGE FREE-PISTON COMPRESSOR FOR COMPRISING GAS SUPPLIED THERETO UNDER AN ADMISSION PRESSURE WHICH MAY VARY, INCLUDING: A RECIPROCABLE FREE-PISTON ASSEMBLY COMPRISED OF: AN ENGINE CYLINDER; LEFT AND RIGHT-HAND ENGINE PISTONS SLIDABLE WITHIN SAID ENGINE CYLINDER; MEANS FOR SUPPLYING GASEOUS FUEL FOR DRIVING THE COMPRESSOR TO SAID ENGINE CYLINDER; EACH OF SAID ENGINE PISTONS BEING COUPLED TO: A FIRST COMPRESSOR STAGE COMPRISING A FIRST DOUBLEFACED PISTON FOR COMPRESSING GAS ON BOTH SIDES THEREOF; AND A SECOND COMPRESSOR STAGE COMPRISING A SECOND PISTON; MEANS FOR FEEDING SAID GAS BEING SUPPLIED THERETO UNDER PRESSURE WHICH MAY VARY TO SAID FIRST COMPRESSOR STAGE; MEANS FOR COUPLING THE OUTPUT OF BOTH SIDES OF SAID FIRST COMPRESSOR STAGE TO SAID SECOND COMPRESSOR STAGE; THE PISTON AREA OF BOTH FACES OF SAID FIRST PIISTON EACH BEING GREATER THAN THE PISTON AREA OF SAID SECOND PISTON CAUSING THE GAS COMPRESSED THEREIN TO EXERT A RESULTANT FORCE ON SAID PISTON AREAS FOR EFFECTING COMPRESSION OF SAID GASEOUS FUEL IN THE ENGINE CYLINDER, AND SIULTANEOUSLY FOR EFFECTING THE INDUCTION STROKE OF SAID SECOND COMPRESSOR STAGE. 